The optically active carboxylic acids shown by formula (I) have been watched with keen interest as raw materials for synthesizing various useful materials, for example, as intermediates for synthesizing physiologically active substances of natural materials and also as liquid crystal materials.
Several methods of asymmetric synthesis have been available for producing such optically active carboxylic acids, for example, (1) a method starting with naturally occurring optically active isomers; (2) a method utilizing microbial asymmetric hydrogenation; and (3) a method involving asymmetric hydrogenation in the presence of a specified catalyst.
In particular, as the process for producing the optically active carboxylic acid of formula (I) from the .alpha., .beta.-unsaturated carboxylic acid of formula (II) by asymmetric synthesis, a process in which the .alpha.,.beta.-unsaturated carboxylic acid is asymmetrically hydrogenated in the presence of a rhodium-optically active phosphine complex as a catalyst is reported. That is, C. Fisher et al report in Tetrahedron Letters, 29, pp. 2487 to 2490 (1977) that 2-methylphenylacetic acid is obtained in an optical yield of 27.5% ee by asymmetric hydrogenation of atropic acid (2-methylenephenylacetic acid). Also, P. Aviron-Violet et al report asymmetric hydrogenation of atropic acid in an optical yield of 70% ee in J. Mol. Cat., 5, pp. 41 to 50 (1979). Furthermore, M. Yamashita et al report in Bull. Chem. Soc. Jon., 55, pp. 2917 to 2921 (1982) that 2-methylbutyric acid is obtained by asymmetric hydrogenation of tiglic acid ((E)-2-methyl-2butenoic acid) in an optical yield of 62% ee.
Among these methods of asymmetric synthesis, according to the method (1) starting with naturally occurring optically active isomers or method (2) utilizing microbial asymmetric hydrogenation, though desired carboxylic acids with high optical purities can be obtained, not only the absolute configuration of the resulting optically active carboxylic acids is limited to a specific one, but also it is difficult to synthesize their enantiomers. Further, in accordance with the asymmetric hydrogenation of .alpha.,.beta.-unsaturated carboxylic acid derivatives using a rhodium-optically active phosphine catalyst, not only the optical purities of the resulting carboxylic acids are not yet satisfactory, but also since metallic rhodium to be used is expensive due to limitations in place and quantity of production when used as a catalyst component, it forms a large proportion in cost of the catalyst, ultimately resulting in an increase in cost of the final commercial products.